I did not realized it was an option.  ​IIRC we had it on the Teklabs 11/60, but we could not run the tools easily so we ended up never messing with it.   We had talked about adding the CMU csav/cret instructs from the 11/40e but we got the 11/70 and that sucked up my time and I never got back to try it.

It was not a redux of the 11/34 BTW.  Jeff Mitchell did the 11/34 and was off working on the 750 when the 60 was done.  I believe Al Pat was somehow involved with 60, but I've forgotten. 

As for the "halt and confuse uCode" stuff.  The 60 was notorious for getting "stuck" when running UNIX.    I've now forgotten many of the details, IIRC it had to with context switch, register save and I think I remember it was something to do with multiple interrupts.   Since Glaser and I did the original 11/60 port, I think we knew most of the USENIX UNIX sites that run it on them since we swapped notes if not code.  All of us used to complain about random hangs where the uCode take a jump and the system would halt.    The only solution when that happened was the pull the circuit breaker power on the back of the machine because the front panel switched were lost.   I personally spend many hours with the DEC guys trying to figure what is was, we could never repeat it.   Years later, when I got to know some of the uCode engineers that had worked on the 11s and the Vaxen, I found out it was agreed at DEC engineer there was a uCode bug, but so few UNIX customer were out there (and the system had gone to "traditional products") management dropped as not being worth finding and fixing.